1. Field of the Invention
The present invention relates to a security fence of the type employing a fiber optic cable to detect intrusion or tampering. Further, the present invention relates to a method and apparatus for calibrating and initializing such a system, so that the system accurately approximates an intrusion or tampering location along the security fence.
2. Discussion of the Related Art
Security fences are widely used today. For example, security fences usually surround the perimeters of military facilities, some government agencies, airports, residences of celebrities and politicians, and other such areas. Simple fences are effective in alerting an innocent passerby that a certain area is restricted. Deterrent fences, such as fences with barbed wire, razor wire, or electrical currents therein, can also be effective at deterring less determined persons, such as children and vagabonds, from crossing into the restricted area. However, determined individuals, such as criminals and terrorists, may easily bypass deterrent fences by using common tools, such as wire or bolt cutters to simply make a passageway therethrough.
A first attempt to address the concern of determined individuals entering a restricted, fenced area was the employment of monitoring schemes. Security guards, cameras, and watch dogs, to name a few, were used to monitor the fence perimeter. However, such conventional monitoring systems are far from foolproof, as humans and animals can be distracted and often do not monitor closely due to boredom.
An improvement in the art came with the advent of employing fiber optic cable in conjunction with a security fence. FIG. 1 shows a section of a security fence 1, in accordance with the background art. The security fence extends from a first end 3 to a second end 5. Such a security fence is formed by interlocked galvanized metal wires attached to support posts 2, and is commonly referred to as a “chain-link” fence. Of course, a protected area would be surrounded by a plurality of such fence sections, which abut, or are closely adjacent to, one another.
A fiber optic cable 7 is woven into an overall pattern and attached to each section of the security fence 1 at a plurality of locations along the section of the security fence 1. FIG. 2 is close-up view of fiber optic cable 7 of the security fence 1. FIG. 2 illustrates the overall weave pattern of the fiber optic cable 7. Six columns and five rows of the weave pattern are illustrated, however in practice, there could be thousands of columns and dozens of rows in a weave pattern covering a complete security fence section 1. The galvanized wires have been removed to simplify the illustration. The fiber optic cable 7 is attached to the security fence 1 by a plurality of clips 9. As illustrated in FIG. 3, the clips 9 connect one portion of the fiber optic cable 7 to another portion of the fiber optic cable 7, and also attach the fiber optic cable 7 to galvanized wires 6 of the security fence 1.
As illustrated in FIG. 2, a light is piped into one end 12 of the fiber optic cable 7 via a source/receiver, known as a transceiver 10 or ODTR. The light passes through the fiber optic cable 7 until it reaches the other end 14 of the fiber optic cable 7. At the other end, the light is reflected off of a termination and returns back to the transceiver 10. In practice, the weave pattern of FIG. 2 would be continuous all over the security fence 1, and the one end 12 of the fiber optic cable 7 would reside at the first end 3 of the security fence 1. Likewise, the other end 14 of the fiber optic cable 7 would reside at the second end 5 of the security fence 1.
The time delay between the transmission of the light and the return of the reflected light is indicative of the length of the fiber optic cable 7. A typical length of the fiber optic cable 7 might be 5,000, 10,000 or even 20,000 meters (m). If the cable is disturbed (e.g. cut by a tool or bent sharply as by climbing), the transmission of light therethrough is interrupted. The interruption causes the transmitted light to be partially or completely stopped before reaching the other end 14 of the fiber optic cable 7, and instead causes the transmitted light to be reflected back to the transceiver 10 from the point of the cut or sharp bend.
The transceiver 10 constantly monitors the time delay between transmitting light and receiving reflected light back. If the measured time delay remains within a threshold value of a standard time delay, indicative of the light reaching the other end 14 of the cable, the transceiver 10 knows that the fiber optic cable 7 remains unmolested (e.g. uncut and unbent). If the time delay varies outside of the threshold value, e.g. less than the standard time delay, the transceiver 10 assumes that an uncommon event has occurred, and an alarm is raised.
Because of the nature of the speed of light and electronic circuits, the alarm is raised at almost the same instant as the breaching of, or tampering with, the fence. However, it should be noted that the length of fence being monitored by the system is usually quite long. For example, one transceiver 10 can monitor a fence up to and perhaps exceeding one mile (1.6 kilometers) in length. In most circumstances, such a fence is too long to be monitored by a person or camera from a single vantage point.
Initially, it is important to gain at least a general idea of the potential breach (PB) point along the fence from the transceiver 10. By knowing the general area of the PB, it is possible to have a quick response by personnel to the area of a PB. Further, it is possible to quickly activate and/or aim a camera to the general area of the PB.
Later, it is also very important to have a more specific idea of the PB point in order to facilitate inspection and servicing of the fiber optic cable 7 to ensure/restore its operability. If the fiber optic cable 7 has been cut, it is important to “know” a location of the cut with some precision, so as to facilitate its timely repair. If a general location of the cut in the fiber optic cable is only known to within plus or minus 30 meters, it can take several people a long time to trace or follow the weave pattern and try to discern the cut or damaged portion of the fiber optic cable 7, so that the cable can be repaired.
To locate a PB, the background art employs an arithmetic approach, as will now be explained. A signal is introduced into the first end 12 of the fiber optic cable 7 and initially travels along the security fence 1 toward a termination at the second end 14 of the fiber optic cable 7. The initial travel direction has been indicated by arrows in FIG. 2. After reaching the termination, the light is reflected at the second end 14, and travels back to the transceiver 10. No arrows for the reflected light are included in FIG. 2, in order to simplify the illustration.
The transceiver 10 monitors the time delay between the transmission of a light signal and the reception of the reflected light signal. The time delay can be converted into a length measurement by multiplying the time delay by the speed of the light transmitted through the fiber optic cable 7 (which is a known value), and dividing that product by two. Under normal circumstances (e.g. no cut or bending stress in the fiber optic cable 7), the distance calculated by the transceiver 10 will be the cable's total length (TL), otherwise the length will be a shorter value and will indicate a length of cable prior to the PB point in the fiber optic cable 7. This length will be referred to as the cut length (CL).
To locate the ground distance (GD) from the first end 3 of the security fence 1 to the potential breach/bend (PB) in the fiber optic cable 7, the transceiver 10 starts with the measured CL, and then subtracts a dummy cable length (DCL), which extends between the transceiver 10 and the start of the security fence 1. Next, the outcome is divided by the cable length used per meter of ground length (CLM). The CLM is an average value, which is highly dependent upon such factors as the shape of the weave pattern selected (which is diamond shaped in FIG. 2), the closeness or density of the pattern, and the height of the security fence 1. In some instances, CLM could equal 25 meters of cable per one meter of ground distance. The equation to estimate the ground distance (GD) from the start of the fence to the potential breach (PB) in the fiber optic cable 3 is: GD=(CL−DCL)/CLM.
Authorized personnel use the ground distance (GD) as a general guide to quickly respond to a potential breach (PB). For example, a security guard would be alerted to a potential break-in at 1,113 meters from the start point of the fence. The guard would then quickly proceed to a point in the neighborhood of 1,113 meters from the start of the fence in an attempt to intercept the breaching party. Later, the service personnel would attempt to exactly locate a point along the fence, which is approximately 1,113 meters from the start point of the fence, so that the fiber optic cable 3 could be inspected and repaired, as needed.
The background art, described above, suffers several drawbacks. First, it is difficult to locate points along a fence line based upon a known distance from a start point of the fence. If the distance is long, it is tedious to measure such a distance, and the measurement is prone to error. Further, obstacles along the fence line can further hinder a measurement from the start of the fence.
Second, the value CLM, which represents an average cable length used per meter of ground length, is a very troublesome value. In order for the ground distance (GD) to be accurately calculated, the CLM must remain relatively constant along the length of the fence. In other words, the actual CLM at any point along the fence should remain at, or very near to, the value of the average CLM for the entire fence, which is used in the equation to calculate the ground distance (GD).
In reality, it is very difficult to maintain a relatively constant CLM along the entire length of the fence line. For example, the height of the fence may vary to accommodate terrain changes. Further, it is difficult, and hence time consuming and expensive, to maintain a constant weave density for the weave pattern of the fiber optic cable 3. Therefore, there exists a need in the art for an improved system and method of calculating a ground distance (GD) to a potential breach (PB) point in a fiber optic cable enhanced, security fence, such as the security fence 1 illustrated in FIG. 1.